Geo-fence refers to a virtual perimeter of a geographic area. Applications of geo-fence include detecting crossing of such geo-fences, for example, for ensuring safety. For example, geo-fence techniques may be used to let parents know if their small children travel outside a neighborhood or other set region, and/or to recover stolen vehicles. Geo-fence techniques may be also used to notify one if someone, e.g., a friend is traveling in the same area. Computer-implemented systems or applications in conjunction with Global Positioning System devices may detect such crossing and provide notifications, for instance, via mobile or another device.